Recently, a light emitting diode (LED) is spotlighted as a light emitting device. Since the LED can convert electric energy into light energy with high efficiency and long life span of about 5 years or more, the LED can remarkably reduce the energy consumption and repair and maintenance cost. In this regard, the LED is spotlighted in the next-generation lighting field.
Such an LED includes a light emitting device semiconductor layer including a first conductive semiconductor layer, an active layer and a second conductive semiconductor layer, in which the active layer generates light according to current applied to the first and second conductive semiconductor layers.
Meanwhile, the LED may be grown from a sapphire growth substrate, and the light emitting semiconductor layer is placed on c (0001) planes vertically crossing each other about a crystal c-axis of the sapphire growth substrate. Symmetry elements contained in a wurtzite crystal structure represents that the single crystal of a group III nitride-based semiconductor has spontaneous polarization along the c-axis. In addition, if the wurtzite crystal structure is non-centrosymmetric crystal structure, single crystals of group III nitride-based semiconductor may additionally represent piezoelectric polarization.
Recently, the growth technology of the single crystal of the group III nitride-based semiconductor employs single crystals of a group III nitride-based semiconductor terminated with a group 3-metal polar surface and grown in a c-axis direction. In other words, when the single crystals of the group III nitride-based semiconductor are grown by MOCVD or HVPE growth equipment, a surface making contact with air represents a group-III metallic polarity, and a surface making contact with the sapphire substrate serving as a growth substrate represents a nitrogen polarity.
Accordingly, if the same electrode material, that is, Ti/Al is stacked on both of the surface of a gallium nitride having a gallium polarity belonging to group-III metal and the surface of a gallium nitride having a nitrogen polarity, contacting interfaces represent different behaviors according to heat treatment temperatures.
Meanwhile, the LEDs are classified into a lateral-type LED and a vertical-type LED. In the lateral-type LED, a first electrode layer is formed on a first conductive semiconductor layer having a group-III metallic polarity surface. In the vertical-type LED, the first electrode layer is formed on the first conducive semiconductor layer having a nitrogen polarity.
Accordingly, when a first electrode layer is formed in the vertical-type LED similarly to the lateral-type LED, a superior ohmic contacting interface cannot be formed, and high driving voltage drop occurs, so that a great amount of heat may be emitted and the life span of the LED may be reduced.